PURPOSE: To investigate alterations in functional connectivity in the resting brain networks in healthy elderly volunteers and patients with mild, moderate, or severe Alzheimer Disease (AD). MATERIALS AND METHODS: This study was approved by the institutional ethics committee, and informed consent was obtained. Forty-six patients with AD and 16 healthy elderly volunteers were prospectively examined. Resting-state functional magnetic resonance imaging was used to detect alterations in posterior cingulate cortex (PCC) functional connectivity through a comparison of the healthy control group with three separate AD groups-mild, moderate, and severe AD. A temporal correlation method was used to obtain PCC connectivity maps. RESULTS: Dissociated functional connectivity between the PCC and a set of regions, including the visual cortices bilaterally, the inferior temporal cortex, the hippocampus, and especially the medial prefrontal cortex and the precuneus and/or cuneus, was observed in all AD groups. The disruption of connectivity intensified as the stage of AD progression increased. There were also regions that exhibited increased connectivity; these regions extended from left lateralized frontoparietal regions and spread to bilateral frontoparietal regions along with AD progression. CONCLUSION: Changes in PCC functional connectivity comprised bidirectional alterations in the resting networks in AD-affected brains, and the impaired resting functional connectivity seemed to change with AD progression. Therefore, alterations in functional connectivity in the default mode network might play a role in the progression of AD.